gluster: Corrected the event log to show old value

Corrected the event log mechanism to show the old value also during the
editing of volume from UI. Earlier the message used to show only new
value being set for an option in the form "Volume Option
<option-name>=<option-value> set on <volume-1>."

Change-Id: I3cc679a70215d1f591cccf12554c3b1e076f49e8
Bug-Url: https://bugzilla.redhat.com/916117
Signed-off-by: Shubhendu Tripathi <shtripat@redhat.com>